In living organisms, biological processes generate different types of signals, which are generally referred to as biosignals. Biosignals may be is electrical, mechanical or chemical.
Bioelectromagnetism is a broad field including both measurements of electromagnetic fields of bioelectric sources and intrinsic properties of tissue. Measurement of the electromagnetic fields of bioelectric sources covers electricity created in life processes internal in tissues, such as excitation of nerve and muscle tissues. In the measurements of intrinsic properties electric currents are supplied from an external source outside living tissues and electric impedance is monitored. Biosignals may thus refer to either active processes or passive properties of the tissue. However, these passive properties can also be related to electrical or other processes internal in tissues, even though the measurement does not directly utilize the electricity generated internally in tissues.
Neuromonitoring is a subfield of clinical patient monitoring focused on measuring various aspects of brain function including changes caused by drugs commonly used to induce and maintain anesthesia in an operating room or sedation in patients under critical or intensive care.
Electroencephalography (EEG) is a well-established technique for assessing brain activity by recording and analyzing the weak bioelectric signals generated in the cortex of the brain using electrodes attached on the skin of the skull surface. EEG has been in wide use for decades in basic research of the neural systems of the brain as well as in the clinical diagnosis of various neurophysiological diseases and disorders.
Electromyography (EMG) is a method for recording bioelectric signals of muscles. In an EMG measurement, the electrodes are attached on top of a muscle onto the surface of the skin. When an EMG signal is recorded from the forehead of the patient, the recorded signal may include both the activity of the facial muscles (fEMG) and of the brain (EEG). As the frequencies of the EMG spectrum are usually high and above the frequencies of the brain activity, the EMG components can be separated by methods of signal processing or spectral analysis from the EEG signal.
Most patients being cared for in an intensive care unit receive some form of sedation. However, the control of the depth of the sedation administered to a patient is still problematic, and therefore oversedation and undersedation are both common occurrences in ICUs. At present, monitoring the level of sedation is mainly handled by using subjective observations from the patient. Various sedation assessment scales have been developed for subjectively assessing the level of sedation, the Ramsay Score being one of the most widely used tools for this purpose. These scoring systems typically assess the different components of the state of the patient, namely motoric and hypnotic components, and the level of agitation. The scores of the components are, however, not mutually independent and therefore reliable assessment of motoric and hypnotic statuses is difficult or impossible.
However, as discussed in the article by P. A. McGaffigan: Advancing sedation assessment to promote patient comfort, Critical Care Nurse/Supplement, February 2002, pp. 29-36, sedation assessment is currently evolving towards a more disciplined and standard part of clinical practice, in which different objective sedation assessment tools are used in order to improve the reliability of the sedation assessment. The need for reliably monitoring the level of sedation is not only based on the desire to improve the quality of the patient care, but also on economy related aspects. As discussed in the above-mentioned article, growing evidence shows that inappropriate sedation can lead to adverse clinical outcomes and reduced efficiencies in critical care settings. Oversedation may cause various complications, such as cardiovascular instability, and it may also increase the length of stay in the hospital and prolong the usage time of expensive facilities, such as the intensive care unit. Undersedation, in turn, may result in patient anxiety and agitation, which can further interfere with care and result in harm to the patient and the nursing staff.
One of the special applications of electroencephalography, which has received attention recently, is the use of a processed EEG signal for objective quantification of the amount of brain activity for the purpose of determining the level of consciousness of a patient. In its simplest form, the utilization of an EEG signal allows for the automatic detection of the alertness of an individual, i.e. if he or she is awake or asleep. This has become an issue of increased interest, both scientifically and commercially, in the context of measuring the depth of unconsciousness induced by anesthesia during surgery. As in the context of sedation, the reasons for the increased interest with respect to anesthesia relate both to the quality of care and to the costs involved. Balanced anesthesia reduces surgical stress and there is firm evidence that adequate analgesia decreases postoperative morbidity. Awareness during surgery with insufficient analgesia may lead to a post-traumatic stress disorder. Low quality pre- and intra-operative analgesia makes it difficult to select the optimal pain management strategy later on. More specifically, it may cause exposure to unwanted side effects during the recovery from the surgery. If the anesthesia is too light and involves insufficient hypnosis, it may cause traumatic experiences both for the patient and for the anesthesia personnel. From an economical point of view, if the anesthesia is too deep, it may cause increased perioperative costs through extra use of drugs and time, and extend the time required for post-operative care.
In connection with anesthesia, the patient is administered hypnotic, analgesic, and neuromuscular blocking agents. The neuromuscular blocking agents block neuromuscular junctions, as a result of which the patient loses the ability to move. Sedatives, in turn, have usually both hypnotic and analgesic properties, but neuromuscular blocking agents are rarely used for sedation.
In addition to EEG signal data, EMG signal data obtained from facial muscles (fEMG) of the forehead is used for monitoring purposes during anesthesia and intensive care. The facial muscles are usually the first indicators of a patient approaching consciousness. When this muscle activity is sensed by appropriately placed electrodes, it provides an early indication that the patient is emerging from anesthesia. Similarly, these electrodes can sense pain reactions when the anesthesia is not adequate due to inadequate analgesia. So, the fEMG signals give an early warning of arousal and may also be indicative of inadequate analgesia.
For defining the level of sedation two different and mutually independent components, namely hypnotic and motoric components, are essential. For assessing the hypnotic state of the patient, EEG signal processing is required, and for assessing the motoric state of the patient, EMG signal processing is normally needed.
An objective tool for assessing the level of anesthesia or sedation is disclosed in International Patent Application WO 02/32305, which describes a method and device for ascertaining the cerebral state of a patient. In this disclosure, a measure derived from EMG signal data enhances and confirms the determination of the hypnotic state made using EEG signal data. As the EMG signal data may be computed more frequently than the EEG signal data, this renders ascertaining changes in the hypnotic state of the patient more rapid. The combined indication provided by the EEG signal data, indicative of the hypnotic component, and EMG signal data, indicative of the motoric component, may also be used for assessing the adequacy of anesthesia or the level of sedation.
Commercially available processes and apparatuses utilizing EEG and EMG signal data for monitoring a patient under sedation or anesthesia rest on a single measurement channel for obtaining the data needed. In other words, the processes utilize the fact that the frequencies of the EMG spectrum are above the frequencies of brain activities, so that the EMG signal component and EEG signal component are obtained from the single channel data by a division of the data.
In order to achieve an accurate measurement of the level of sedation, the indicators indicative of the motoric and hypnotic states should therefore be orthogonal, i.e. mutually independent. However, as the spectra of the EEG and fEMG signals overlap, the discrimination of these two signal components in the single measurement channel requires sophisticated algorithms and optimal electrode position on the forehead of the patient.
The present invention seeks to alleviate or eliminate the above-mentioned drawbacks and to provide a method and apparatus by means of which the accuracy of the measurements may be improved in environments of the above kind, i.e. one in which different biosignal components are relevant in order to obtain a assessment result for a physiological condition of a patient.